1. Field of the Invention
The present invention relates to a method and an apparatus for magnetic resonance (MR) system, particularly for simultaneously acquiring multi-slice/slab magnetic resonance imaging (MRI) signals.
2. Description of the Prior Art
a. Principle of Two-Dimensional Magnetic Resonance Imaging
The principle of two-dimensional (2D) MRI procedure is described as follows: As a subject is placed in a static magnetic field, a region of the subject can be excited by using a radio-frequency (RF) coil and giving signals with respect to all the excitation and relaxation of nucleus excitations and relaxations in the region. With a (magnetic) gradient applied, the RF coil can receive those signals, which can be processed to a MR image. If the change in the structure or functionality of the region is to be realized, the gradient may be adjusted so that slices can be acquired from various locations in the region.
In the following, 2D spatial encoding is described. In the beginning, a slice-selection gradient GZ is turned on and a slice normal to the Z-direction is excited by RF pulses with suitable frequencies. A phase-encoding gradient GY is then turned on for a period of time and then is turned off, so that those nuclei have a certain phase difference in the Y-direction. As a result, a frequency-encoding gradient GX is turned on, while starting to receive signals. Due to this phase difference, the sum of signals with different frequencies (in the X-direction) is received, giving a line in a space spanned by two variables, i.e., the phase difference and the frequency. This space is termed the k-space. A similar procedure may be repeated, where the phase-encoding gradient GY is changed and thus those nuclei have another certain phase difference in the Y-direction. At this phase difference, the sum of signals with different frequencies (in the X-direction) is received, giving another line in the k-space. The whole sampling in the k-space is completed as the sums of signals with different frequencies are received at different phase differences. When the phase difference is changed, the slice-selection gradient GZ is turned on to excite the selected slice.
A 2D Fourier transform is performed after the k-space sampling is completed. That is, the phase differences and the frequencies are transformed into signal intensities at locations on the XY-plane, forming an image of a horizontal slice or a slab. Thus, a 2D MR image is made.
b. Principle of Three-Dimensional Magnetic Resonance Imaging
The principle of the 3D MRI is similar to that of the 2D MRI, except for a difference in the spatial encoding. A phase-encoding gradient GY and a slab-selection gradient GZ are turned on for a period of time and then are turned off, so that those nuclei have certain phase differences in the Y-direction and Z-direction. A similar procedure may be repeated, where the phase-encoding gradient GY and the slab-selection gradient GZ are changed. A 3D Fourier transform is performed after the k-space sampling is completed, giving a 3D MR image.
c. Common Drawback of Two or Three-Dimensional Magnetic Resonance Images
Referring to FIG. 1, in a conventional procedure of 2D spatial encoding, only one single slice of the subject can be processed at a time; that is, multi-slice images are acquired from multiple scans along a scan direction. Therefore, one image is obtained from one scan; N image is obtained from N scan. The time required for acquiring the images of all the slices can be calculated as Equation 1 (Eq. 1).
Time required for acquiring the images of all the slices=NEX×Npe×TR×Nslice,  (Eq. 1)
where NEX denotes the average number of signaling and Npe is the whole number of encoding. For the 2D MR image, Npe denotes the number of phase encoding Np, TR denotes the time required for acquiring a line in the k-space, and Nslice denotes the number of the slices. For example, if there are total 256 images to be acquired, then Nslice=256, NEX=1, Npe=128, TR=0.1 second, and the time required for acquiring the images of all the slices is about 54 minutes. This is therefore a time-consuming procedure.
Referring to FIG. 2, in a conventional procedure of 3D spatial encoding, in one scan only one single slab of the subject can be excited to give the images of all the slices. Also, the time required for acquiring the 3D MR is calculated as Eqn. 1, but the whole number of encoding Npe=the number of phase encoding Np×the number of phase encoding NZ. Thus, it is apparent that more time is required for acquiring the 3D MR image.
d. Other Related Techniques
MRI is a useful tool for biomedical applications to obtain real-time images. Any possible method to accelerate the MRI scan time is highly attractive. Thus, a great deal of manpower and resources have been invested in this research field, resulting in the development of various ways of acceleration such as simultaneous excitation-time division multiple acquisition, phased array coil acceleration, and reduction in data reception.
The following list some of the approaches developed to date:
i. Simultaneous multi-slice acquisition of MR images by Hadamard encoded excitation (called SIMA sequence for short). Multiple slices are excited simultaneously, but a composite image of the multiple slices is received. The characteristic of the RF electromagnetic wave excited from each slice is required for solving a certain polynomial to give the complete information of an individual image. This technique has the disadvantage that extra time for computation may be needed and that N excitations and N receptions are required for computing the information of N slices.
ii. Simultaneous parallel inclined readout image technique (called SMA sequence for short). In this approach, multiple slices are excited simultaneously, and in receiving, different magnetic field intensities are applied to different slices by means of the gradient coil. This technique has the disadvantage that the additional gradient coil may result in inevitable blur in the image and may thus reduce the image quality.
iii. Use of multi-coil array for the separation of signals from multiple slices simultaneously excited (called SENSE sequence for short). In this approach, the images of different slices are received simultaneously using the sensitivity difference of multiple coils at different locations. The real images with respect to different locations are then computed. This technique has the disadvantage that extra coils are needed; for example, four times acceleration requires at least four coils. The acceleration effect is not proportional to the amount of coils.
iv. MAMBA. One stepped gradient coil is added, besides common linear gradient coils, so that the images of different slices have different resonant frequencies. This technique has the disadvantage that extra coils are needed (MAMBA gradient coils) and the acceleration factor and the multi-slice pitch are unchanged due to the fact that the coils cannot be arbitrarily adjusted.
Therefore, whether simultaneous excitation-time division multiple acquisition or phased array coil acceleration is used, conventional multi-slice image processing techniques cannot, at several times acceleration, simultaneously excite and acquire multi-slice, real-time images under the same pulse time-sequence without adding coils or computation algorithms, or reducing the acquired data. Moreover, these conventional multi-slice/slab MR image processing techniques require additional costs for adding coils or computation algorithms.